(a) Field of the invention
The present invention relates to a wide angle zoom lens system lending itself well fit for use with cameras which are limitless in terms of their back focal distances, like "lens shutter cameras" and, more particularly, to a ternary negative-positive-negative type of wide angle zoom lens system which has an extended zoom range toward the wide angle side so that it can have a field angle of 70.degree. or more at the wide angle end and a zoom ratio of 3 or more.
(b) Description of the Prior Art
So far, many types of zoom lens systems have been known for use with the lens shutter cameras which are limitless in terms of their back focal distances, or other cameras. Of these systems, however, the binary positive-negative type of zoom lens systems are found to have a zoom ratio of at most 2. There are also available ternary zoom lens systems having a zoom ratio of about 3, which are represented by such a ternary positive-positive-negative type of zoom lens system as set forth in Japanese Patent Laid-Open Publication No. 63 (1988)-153511, such a ternary negative-positive-negative type of zoom lens system as described in Japanese Patent Laid-Open Publication No. 64(1989)-72114 and such a ternary positive-negative-positive type of zoom lens system as disclosed in Japanese Patent Laid-Open Publication No. 2(1990)-71220. Japanese Patent Laid-Open Publication No. 2(1990)-37317, on the other hand, proposes a zoom lens system which exceeds 3 in terms of zoom ratio and has a zoom ratio extended toward the wide angle side by reducing its focal length on the wide angle side.
A problem with the binary type of zoom lens system, however, is that when it is intended to extend the zoom ratio to about 3, there is an increase in variations of the magnification of the second lens group; much difficulty is encountered in correcting aberrations because of aberrational fluctuations increased by zooming.
With the ternary type of zoom lens systems, it may be possible to extend the zoom ratio to about 3. However, when the first lens group is of positive refracting power, as is the case with the positive-positive-negative or positive-negative-positive system, it is difficult to form a retrofocus design at the wide angle side. When it is intended to extend the zoom ratio toward the wide angle side, therefore, it is impossible to have sufficient back focal distance, thus causing the off-axial bundle of rays to pass through the 3rd lens group at a position far off the optical axis. This results in an increase in the diameter of the 3rd lens group and renders it difficult to correct off-axial aberrations. In addition, because the associated film surface is located too closely to the surface of the last lens, it is likely that dust deposits on the surface of the last lens may be transferred onto the film, or ghost rays may be emitted by the reflection of light off the film and the surface of the last lens.
By contrast, the ternary negative-positive-negative type of zoom lens system can have sufficient back focal distance at the wide angle side, because it is easy to obtain a retrofocus design at the wide angle side. However, a problem with this type is that difficulty is involved in obtaining a telephoto design at the telephoto side, thus giving rise to an increase in the overall length on the telephoto side. The zoom lens system referred to in the above-mentioned Japanese Patent Laid-Open Publication 2-37317--wherein the zoom ratio is extended by reducing the focal length at the wide angle side to 28.8 mm--also presents a problem that it is increased in the number of lenses involved, with the resulting increase in the overall length at the telephoto side.